The present invention relates to a phenolic resin-based adhesive composition which is excellent in adhesive strength, especially adhesive strength after heating, and operability, and which does not generate volatile solvents or waste water, and to a process for producing the same.
Conventional phenolic resin-based adhesives are generally those of solvent type which are prepared by dissolving, in solvents such as alcohols or ketones, phenolic resins (novolak type phenolic resins and hexamethylenetetramine or resol type phenolic resins) or mixtures of phenolic resins with polyvinyl butyral, polyvinyl acetate or the like, and those of aqueous type which are prepared by emulsifying or completely water-solubilizing them (the solvent type or aqueous type phenolic resin adhesives are disclosed, for example, in xe2x80x9cAdhesive Data Bookxe2x80x9d pages 13-18, edited by Japan Adhesive Society and published from Nikkan Kogyo Shinbun-sha, in 1990; and xe2x80x9cAdhesive Application Techniquesxe2x80x9d pages 218-219, edited by Limited Company N.N. and issued by Nikkei Gijutsu Tosho Co., in 1991). Use of these solvent type and aqueous type phenolic resin-based adhesives needs to coat the adhesives on materials to be bonded, dry the coat and hot-press the materials. The drying step requires a long time to volatilize the solvent or water, resulting in deterioration of operability, and furthermore the volatilized solvent or water must be recovered or abandoned.
On the other hand, powder type phenolic resin-based adhesives which require no drying step and produce no industrial wastes, have been investigated. When the novolak type phenolic resins are used as such adhesives, in many cases, they are mixed with pulverized hexamethylenetetramine (see, for example, xe2x80x9cAdhesive Handbookxe2x80x9d pages 250-251, edited by Japan Adhesive Society, published from Nikkan Kogyo Shinbun-sha, in 1971), and a uniform coat cannot be formed in coating of the adhesives on materials to be bonded, and no sufficient adhesive strength can be obtained. They suffer from further problems that adhesive strength of the adhesives after being heated is not sufficient, and the adhesive strength deteriorates upon exposure to thermal history in use environment to cause separation of the bonded materials.
The present invention has been accomplished as a result of intensive research conducted in an attempt to solve the above problems of the conventional phenolic resin-based adhesives, and the object of the present invention is to provide phenolic resin-based adhesives which are excellent in adhesive strength, especially adhesive strength after being heated, and operability, and which do not produce volatile solvents or waste water.
The present invention relates to a phenolic resin-based adhesive composition which contains a polyvinyl butyral, a novolak type phenolic resin or a novolak type phenolic resin and a resol type phenolic resin, and hexamethylenetetramine and which is prepared by melt-mixing these components, and to a phenolic resin-based adhesive composition which contains a polyvinyl butyral, an epoxy resin, a novolak type phenolic resin or a novolak type phenolic resin and a resol type phenolic resin, and hexamethylenetetramine and which is prepared by melt-mixing these components.
Preferred is a phenolic resin-based adhesive composition which contains a polyvinyl butyral, an epoxy resin, a novolak type phenolic resin, a resol type phenolic resin, and hexamethylenetetramine and which is prepared by melt-mixing these components. Furthermore, a phenolic resin-based adhesive composition which is excellent in operability and adhesive strength, especially adhesive strength after being heated, can be obtained when the hexamethylenetetramine as a hardener forms an adduct with the phenolic resin.
That is, it has been confirmed that a composition containing an epoxy resin, a novolak type phenolic resin and a resol type phenolic resin as essential components is effective for improvement of adhesive strength after being heated.
The present invention will be explained specifically below.
In the present invention, a polyvinyl butyral is added to improve peeling strength and adhesive strength under shear at room temperature of the phenolic resin-based adhesives. Its polymerization degree is preferably 200-3000. If the polymerization degree is less than 200, sufficient adhesive strength cannot be obtained, and if it exceeds 3000, viscosity of the adhesives at melting increases and it is sometimes difficult to obtain a uniform adhesive coat. Content of the polyvinyl butyral in the phenolic resin-based adhesive compositions of the present invention is 1-50 parts by weight, preferably 5-30 parts by weight based on 100 parts by weight of the phenolic resin. If the content is less than 1 part by weight, sufficient adhesive strength cannot be obtained, and if it exceeds 50 parts by weight, viscosity of the adhesives at melting increases and it is difficult to obtain a uniform adhesive coat.
The epoxy resins used in the present invention are those which have at least two epoxy groups in one molecule, and which may be solid or liquid at room temperature. Examples of the epoxy resins are those of bisphenol A type, bisphenol S type, phenol novolak type, cresol novolak type, biphenyl type, naphthalene type and aromatic amine type, which do not limit the epoxy resins used in the present invention. These can be used each alone or in combination of two or more. In the present invention, the epoxy resins are added to improve adhesive property of the phenolic resins. Content of the epoxy resins is 1-50 parts by weight, preferably 3-30 parts by weight based on 100 parts by weight of the phenolic resin. If the content is less than 1 part by weight, sufficient effect to improve the adhesive property cannot be obtained, and if it exceeds 50 parts by weight, curability of the adhesives deteriorates and sufficient adhesive strength cannot be obtained.
In the present invention, the phenolic resins are excellent in heat resistance, superior in adhesive force, and especially great in the action to absorb thermal stress at high temperatures, and as a result, have an effect to increase adhesive strength at high temperatures. The phenolic resins include novolak type phenolic resins and combinations of novolak type phenolic resins and resol type phenolic resins. Preferred are novolak type cresol-modified phenolic resins or combinations of novolak type cresol-modified phenolic resins and resol type cresol-modified phenolic resins, since they are superior in adhesive strength, particularly adhesive strength after being heated. These phenolic resins can be used in any form of liquid, solid or powder. When a novolak type phenolic resin and a resol type phenolic resin are used in combination, ratio of them is preferably 100 parts by weight or less of the resol type phenolic resin for 100 parts by weight of the novolak type phenolic resin. If amount of the resol type phenolic resin exceeds 100 parts by weight, there is caused the problem that the powdered resin is apt to absorb water or it is apt to aggregate.
As to the hardener for the novolak type phenolic resins, hexamethylenetetramine which is generally used as the hardener for novolak type phenolic resins is sometimes not used because when a resol type phenolic resin is used in combination, this also has an action as a hardener. However, in the present invention, hexamethylenetetramine is used as the hardener. Amount of the hexamethylenetetramine is 3-20 parts by weight, preferably 7-17 parts by weight based on 100 parts by weight of the novolak type phenolic resin, though it depends on the amount of the resol type phenolic resin. If the amount is less than 3 parts by weight, the adhesive hardens insufficiently, and if it exceeds 20 parts by weight, it becomes difficult to obtain a uniform adhesive layer due to the decomposition gas of hexamethylenetetramine.
When only the novolak type phenolic resin is used, similarly hexamethylenetetramine is used, and amount thereof is 3-20 parts by weight, preferably 7-17 parts by weight based on 100 parts by weight of the novolak type phenolic resin.
The process for producing the phenolic resin-based adhesive composition according to the present invention comprises uniformly melt-mixing the phenolic resin with the polyvinyl butyral, hexamethylenetetramine and, if necessary, the epoxy resin. According to a preferred example, a given amount of novolak type phenolic resin or novolak type phenolic resin and resol type phenolic resin, epoxy resin if necessary, polyvinyl butyral and hexamethylenetetramine are charged in a pressure type kneader and melt-mixed under pressure. Temperature at the mixing is suitably a temperature at which the phenolic resin melts, but does not begin to harden. Suitable pressure type kneaders are roll type kneader, pressure kneader, twin-screw extruder, single-screw extruder, etc. When the polyvinyl butyral is dispersed in the phenolic resin, viscosity of the resin abruptly increases, and hence it is difficult to uniformly disperse the polyvinyl butyral using a usual phenolic resin reaction vessel. However, by using the pressure type kneader, it becomes possible to uniformly disperse it in the phenolic resin. Furthermore, it is difficult to uniformly disperse hexamethylenetetramine in the phenolic resin and allow hexamethylenetetramine to form an adduct with the phenolic resin in a usual phenolic resin reaction vessel because of increase in viscosity of the resin or starting of gelling reaction. However, by using the pressure type kneader, it becomes possible to uniformly disperse hexamethylenetetramine in the phenolic resin and allow the hexamethylenetetramine to form an adduct with the phenolic resin.
The hexamethylenetetramine which forms an adduct with the phenolic resin as defined in the present invention means hexamethylenetetramine which cannot be extracted with deionized water. The temperature of water here is 25xc2x11xc2x0 C. Usually, hexamethylenetetramine which has been merely pulverized and mixed with the phenolic resin can be easily extracted with water, but hexamethylenetetramine which has formed an adduct, that is, has formed an intermolecular adduct with the phenolic resin, cannot be extracted with water. Thus, the adduct formation rate of hexamethylenetetramine (proportion of hexamethylenetetramine which has formed the adduct) can be obtained by the following formula.
Adduct formation rate (%)=[(Axe2x88x92B)/A]xc3x97100
A: Total amount of hexamethylenetetramine.
B: Amount of hexamethylenetetramine extracted with deionized water.
A can be obtained by Kjeldahl method, liquid chromatographic method, elementary analysis method, or the like. B can be obtained by titration method on the amount of hexamethylenetetramine extracted with deionized water. Particle size of the sample in the measurement is 150 xcexcm or less, and if the particle size is coarser, the sample is pulverized to a particle size of 150 xcexcm or less and then the measurement is conducted.
The phenolic resin-based adhesive composition of the present invention is characterized by being obtained by melt-mixing a novolak type phenolic resin or a novolak type phenolic resin and a resol type phenolic resin, a polyvinyl butyral, and hexamethylenetetramine; or melt-mixing a novolak type phenolic resin or a novolak type phenolic resin and a resol type phenolic resin, an epoxy resin, a polyvinyl butyral and hexamethylenetetramine. Especially, when they are mixed with heating by a pressure type kneader, it becomes possible to uniformly disperse hexamethylenetetramine in the phenolic resin and form an adduct of hexamethylenetetramine with the phenolic resin. Furthermore, when the polyvinyl butyral is melt-mixed with the phenolic resin, there is the possibility that viscosity of the resin increases and they cannot be mixed in such a reaction vessel as used for the reaction of a usual phenolic resin. In this case, by using the pressure type kneader, it becomes possible to uniformly disperse the polyvinyl butyral in the phenolic resin.
The phenolic resin-based adhesive composition of the present invention exhibits good adhesive force in bonding of the same or different materials such as metals, papers, woods, and plastics, and is especially suitable for bonding of a phenolic resin molded article with a metal.